Category: Broadcast Radio

Measuring Broadcast FM Multipath Distortion with an RTL-SDR

Over on GitHub user jj1bdx has just released a new tool called rtl_power-fm-multipath which can be used for estimating broadcast FM multipath distortion with an RTL-SDR. Broadcast FM multipath is caused when a signal is received from multiple directions due to it reflecting off and refracting through physical objects like buildings and terrain. As the reflected/refracted signals will be delayed it can cause echo like distortions in the RF signal which can cause issues like poor digital decoding, poor FM audio reception and ghosting in analogue video.

The multipath distortion estimation method works by measuring the ratio of the strength of direct/reflected radio waves which results in the desired/undesired (D/U) ratio. This measurement method was proposed by Komiya87 and JushinFM who both wrote papers in Japanese describing the method. In summary the methodology is:

  • Measure the maximum peak strength for +-50kHz spectrum of the target FM station
  • Obtain the maximum value (Lmax) and minimum value (Lmin) within the spectrum
  • Obtain the ratio of the maximum and minimum values L = Lmax / Lmin (note: Lmax and Lmin are real values (not in dB), and L must be > 1)
  • The estimated D/U ratio R = (L+1) / (L-1) (in the real value, not in dB)

The rtl_power-fm-multipath program is based on rtl_power and works by using rtl_power to record power measurements for 5 minutes, then sending the data to a peakhold function which computes the maximum power value for each frequency, and then calculations the distortion ratio.

An example of Multipath Distortion on a DAB+ Signal. From Gough Lui's post https://goughlui.com/2015/03/28/trip-to-hk-cn-2014-part-5-rtl-sdr-more-radio-ais-night-photos/
An example of Multipath Distortion on a DAB+ Signal. From Gough Lui's post at https://goughlui.com/2015/03/28/trip-to-hk-cn-2014-part-5-rtl-sdr-more-radio-ais-night-photos

Archiving Shortwave History with Software Defined Radio

Broadcast shortwave radio is not always archived for long at the station, so finding sound bites from interesting historical events can be difficult. We know that songs are of course recorded, but talk back radio, discussions in between music, news readings, weather updates, ads and pirate radio are all lost over time. Although these things may seem mundane now, future historians may be interested in listening in on this little slice of life.

At this years HOPE XIII conference, Thomas Witherspoon, owner of the popular SWLing Blog gave a talk titled "Creating a Radio Time Machine: Software-Defined Radios and Time-Shifted Recordings". Currently the talk is available as a full recording of all talks at the conference over on Livestream. Thomas' talk begins at about 9h03m45s (thanks to Aaron Kuhn from the comments for finding the talk. Hackaday also recently ran an article on the content of his talk.

Thomas' idea is to create a database of shortwave radio IQ recordings so that they can be archived for historical purposes. The project is called "The Radio Spectrum Archive" and has a website set up at spectrumarchive.org. To do this modern software defined radios like the RTL-SDR can be used to record a large bandwidth, however the problem is with data storage as IQ recordings can take up extremely large amounts of disk space. 

Interestingly, it turns out that people have actually been making IQ recordings since the 1980's by connecting their shortwave radios to VCR tape recorders. In the modern day these VCR recordings can be digitized into an IQ file, and played back in software like HDSDR. In the video below Thomas demonstrates the playback of a digitized VCR radio recording from May 1 1986. You can hear some interesting news tidbits on the soviet cover-up of Chernobyl, the Challenger disaster and the launch of a new hurricane tracking satellite. If recording was more popular it would have been interesting to hear soviet radio during this time too.

In addition to archiving IQ files, Thomas has been releasing a podcast of curated historical audio recordings from VCR tapes, as well as modern recordings that may be of interest over at shortwavearchive.com.

We envision a future where one day these recordings could be automatically turned into text logs via advanced speech to text software, so they could easily be searched through.

[Also seen on Hackaday]

Radio Spectrum Recordings: A short demonstration

An Update on the PantronX Titus II SDR

The PantronX Titus II is a yet-to-be-released portable Android tablet based SDR that we've been following since 2016. The device will feature a 100 kHz - 2 GHz tuning range, and software that focuses on HF digital DRM decoding, as well as DAB on VHF. 

Thomas from the excellent SWLing blog got curious about the Titus II as he had not heard any updates from the team in a while, so he emailed them requesting an update. Mike from PantronX wrote the following reply:

As you might be aware, we have joined up with Fraunhofer to include their MMPlayer app standard on Titus–what a difference a professional decoder, for both analog, DRM(+), and DAB(+), makes! MMPlayer is full featured even including reliable one way file downloads with DRM.

We are attempting also to license HD to include on the app for North America, making a truly worldwide receiver. Some deficiencies in our version of Android have caused issues as well as MMPlayer. All of which have caused delays leading to some serious business decisions – as you can imagine. You are correct that broadcasters have made large orders that will be fulfilled first. There are units in the field testing and such and continuing resolution of the software issues.

One of the issues that folks seem to have a hard time understanding is that we can not just build a few hundred or even thousands of units. Our minimum run is 10,000pcs! To do that everything has to be 100% – including the software. We simply will not ship units that are not 100%. Titus works, MMPlayer works – its that last 5% that takes the most time to resolve. These facts preclude any incremental production attempts. All that being said, we are very hopeful that the first production run is ready by last quarter of this year.

The Titus II
The Titus II

Hacker Warehouse Demonstrates Pager Decoding with an RTL-SDR

Over on YouTube the web show Hacker Warehouse have created a video explaining wireless pagers and how RTL-SDRs can be used to sniff them. In the video host Troy Brown starts by explaining what pagers are and how they work, and then he shows how to decode them with SDR# and PDW. We have a tutorial on this project available here too.

Later in the video he shows some examples of pager messages that he's received. He shows censored messages such as hospital patient data being transmitted in plain text, sports scores, a memo from a .gov address claiming allegations of abuse from a client, office gossip about a hookup, a message about a drunk man with a knife, a message from a Windows server with IP address and URL, a message from a computer database, and messages from banks.

In the past we've also seen an art installation in New York which used SDR to highlight the blatant breach of privacy that these pager messages can contain.

Decoding Pager Data with RTLSDR - Tradecraft

Video Demonstrating the iBiquity HD Radio Decoder

Over on YouTube user Rob Fissel has uploaded a nice video that demonstrates the iBiquity HD Radio decoder working with an RTL-SDR. HD Radio is a terrestrial digital broadcast signal that is only used in North America. It is easily recognized by the two rectangular blocks on either side of a broadcast FM station signal on a spectrum analyzer/waterfall display.

For a long time it was thought impossible to decode due to the closed and proprietary nature of the signal format. But thanks to Theori who was able to reverse engineer and create an HD Radio decoder it has now become possible to decode this into actual audio that you can listen to. In some areas it is even possible to extract the weather and traffic data encoded into some broadcasts from iHeartRadio.

Rob's YouTube video demonstrates him downloading and setting up the HD Radio decoder, then receiving, decoding and listening to some HD Radio stations in his area.

iBiquity HD Radio - RTL-SDR

Part III of Calibrating your RTL-SDR with QIRX SDR Now Uploaded

Earlier in the month we posted about Clem's three part series on how his QIRX SDR software can be used to accurately calibrate an RTL-SDR using DAB signals. Back then only parts I and II had been released. Now part III has been released, and in the article he explains why it is beneficial to calibrate the frequency error, and also the sampling rate when attempting to decode DAB signals..

Clem begins by explaining how DAB signals work and why it is important to have accurate frequency calibration when receiving DAB. Later he goes on to explain the effect of sampling rate errors due to frequency inaccuracy on received DAB signals. He shows the effect of gradually increasing the sample rate error on the ability of the algorithms to decode DAB signals.

Comparing constellations with sampling rate error correction turned on and off.
Comparing constellations with sampling rate error correction turned on and off.

The Lego Pi RTL-SDR FM Radio

Thank you to RTL-SDR.com reader 'JJ' for writing in with a submission for his Lego Pi Radio. JJ's Lego Pi Radio consists of a Raspberry Pi and RTL-SDR and is designed to be an FM Radio, MP3 and internet radio player all in one, with a cute enclosure made out of Lego bricks. The radio is controlled by an external numpad which allows for a number of presets to be chosen from.

The internet radio and MP3 players are handled in software by VLC player and a script written by JJ is used to map the numpad to RTL-SDR FM presets, or MP3 and internet radio functions. The whole unit is run headless and if anything needs to be updated such as internet radio links, JJ simply accesses the unit via an SSH shell. JJ also writes how he had to try 3 different brands of speakers before he found one that could be driven directly from the Pi with adequate sound quality. In the future he hopes to add a bluetooth remote.

One problem that JJ found was that the standard rtl_fm did not produce high quality audio. Fortunately he found the NGSoftFM software which is capable of outputting high quality FM stereo sound and is compatible with RTL-SDR dongles.

In the past we've seen a similar project that was implemented on a BeagleBone Black. The idea in that project was to switch between FM and internet radio depending on the reception quality.

Lego Pi Radio 2

Using QIRX SDR and DAB Signals to Calibrate RTL-SDR Dongles

Over on his site, Clem the author of the QIRX SDR software package has written up a three part series where he explains an ultra-fast and very accurate method for calibrating the frequency offset of RTL-SDR receivers by using DAB signals. If you are unfamiliar with DAB, it stands for 'Digital Audio Broadcast' and is a type of digital radio station available in multiple countries in the world, especially in Europe. However it is not used in the USA. Clem writes:

I wrote a three-part tutorial about an ultra-fast, generally available (where you have DAB reception) and very accurate method to calibrate RTL-SDR receivers. It is called "Tutorial: Calibrate your RTL-SDR in 15 Seconds", http://softsyst.com/QIRXCalibrate?sequenceNo=0. It is using the frequency of a DAB transmitter as the reference signal, and is coming in three parts:

· Part I: Method and Measurement, describes the method (example) and compares it to two other, well-known methods.

· Part II: Checks, Frequencies, Sampling Rates: Tells how to make plausibility checks on the obtained calibration result, goes into the foundation of different measuring methods, and explains why calibrating a receiver is generally beneficial, not only for DAB purposes (where at least the frequency correction is mandatory).

· Part III: Improving DAB, Tells why it is advantageous for DAB reception not only correcting the frequency, but also the sampling rate (which is often omitted).

Part I and Part II of these are already on our website, Part III will come soon.

QIRX Being used to Calibrate an RTL-SDR dongle on DAB signals
QIRX Being used to Calibrate an RTL-SDR dongle on DAB signals